Vertebrate embryologists and developmental biologists are becoming increasingly interested in a new model system, the zebrafish, Brachydanio rerio. The interest in zebrafish is derived from its value as both a genetic and embryological organism. It is ideal for genetics because the animals are small, hardy and easy to raise in large numbers because: the females lay 100-200 eggs each spawning, which can occur 1-2 times per week, and techniques for mutagenesis and transgenesis have been well- defined. For embryological studies, their advantage is that fertilization is external, and the embryos are nearly transparent, so that the migration of cells and the earliest stages of organogenesis can be directly observed. Many mutant fish have been isolated after using chemical, radiation, or insertional mutagenesis, and at least three mutagenesis screens are currently underway with the goal of reaching saturation within the next few years. The zebrafish is the only vertebrate with these advantages. Specifically we propose to: l) Construct the first complete genetic linkage map of the zebrafish. Our goal is to have a map with at least a 10 cM resolution. 2) Develop a reference cross (1000 zebrafish), and map this cross. This reference cross is essential for initiating fine structure mapping, and facilitating positional cloning. The reference cross will be made available to the entire zebrafish community. 3) Establish a means of resource distribution, insuring marker information is accessible to investigators, and that DNA from the reference cross is easily obtainable. What would the map provide and is this the time to invest in building a genetic linkage map in zebrafish? A map would 1) allow synteny conservation mapping; 2) allow complete integration of a single map, and reference cross for mutations discovered by the zebrafish community; and 3) build the foundation for gene identification and positional cloning. The zebrafish field is rapidly expanding, and there are hundreds of investigators now working on zebrafish and specific zebrafish mutations. If map construction is delayed, not only will funds be expended by several laboratories to make rudimentary independent maps of specific regions, but funds will also have to be allocated to integrate the various "local" maps. For these reasons, and after discussion, the largest zebrafish facilities decided to focus on construction of genetic linkage map using simple sequence length polymorphisms (SSLPs) at the MGH. It is for this reason funds are requested.